In the operation of compressors, and particularly refrigeration compressors, it is very undesirable to start the compressor under load. Most refrigeration compressor equipment provides for a gradual reduction in the pressure on the high pressure side after the compressor has been inoperative for a period of time. The bleed down or reduction in high pressure allows for the subsequent restarting of the compressor against little or no pressure. If a compressor is stopped, and then restarted while the high pressure still remains, the starting torque required and the related energy drain is detrimental to the compressor and its motor. Also, there is the possibility of physically damaging the compressor by the sudden application of additional pressure when a high pressure already exists.
In order to overcome the detrimental aspects of starting a compressor under load, most refrigeration type systems warn the users not to restart the compressor for a minimum period of time. This minimum period of time normally runs in the order of three to five minutes. This warning is many times overlooked and equipment can be damaged. Also, if there is a momentary power failure, the refrigeration equipment may stop and then automatically be restarted on the restoration of power. This type of power failure occurs quite often in residential areas, and is very detrimental to the life and operating characteristics of residential compressors, particularly equipment such as central air conditioning systems. Also in connection with central air conditioning equipment, the occupants of the air condition space many times are known to reset the control thermostat. This resetting may accidentally call for the operation of the compressor in the air conditioning system only a short period after it has stopped and thereby possibly damage the compressor, as well as cause excessive dimming of lights due to the high starting current required to restart the compressor under load.
In prior art systems, this deficiency has been overcome by providing a minimum off-time delay arrangement in the form of a mechanical timer or an electronic timer that responds to the current being drawn by the compressor itself, or by an arrangement in the form of a separate minimum off-time circuit located near the contactor and responsive to voltage to the contactor. The first arrangement requires actually physically and electrically connecting into the line voltage circuits that energize the electric motor that in turn drives the compressor. This type of minimum off-time arrangement is costly and requires very skilled installation. The second, the separate minimum off-time circuit, requires mounting and wiring an additional component also. These two drawbacks, therefore, have limited the practical use of minimum off-time control devices with residential and other relatively small refrigeration compressor arrangements.